Using Wood Ash in the Garden

If you have a fireplace or wood stove that has helped your family stay warm this winter, what do you do with the leftover wood ashes? They can be spread in the garden, but a retired Purdue Extension expert wrote that you shouldn’t overdo it, especially if you haven’t had your garden soil tested for pH recently.

Rosie Lerner, retired Purdue Extension consumer horticulture specialist, wrote that many gardening books advise throwing these ashes in the garden.

“Wood ash does have fertilizer value, the amount varying somewhat with the species of wood being used,” she said. “Generally, wood ash contains less than 10 percent potash, 1 percent phosphate and trace amounts of micro-nutrients such as iron, manganese, boron, copper and zinc.” She added that trace amounts of heavy metals such as lead, cadmium, nickel and chromium also may be present. Wood ash does not contain nitrogen.

She said that the largest component of wood ash (about 25 percent) is calcium carbonate, a common liming material that increases soil alkalinity. “Wood ash has a very fine particle size, so it reacts rapidly and completely in the soil,” she said. “Although small amounts of nutrients are applied with wood ash, the main effect is that of a liming agent.”

So, does your garden need lime? How do you know? The only reliable way to determine that is with a soil test from a certified laboratory. Have you had your garden soil tested recently?

The pH scale is from 0 to 14, with 7.0 being neutral. A soil test reading below pH 7.0 is acidic. A reading above pH 7.0 is alkaline.

The downside of applying too much wood ash to a garden with an already high pH (alkaline) is that your garden plants will begin to suffer from nutrient deficiencies. Your soil may not be deficient in those nutrients, but at higher pH levels, the nutrients are unavailable to plants.

“Nutrients are most readily available to plants when the soil is slightly acidic,” said Lerner. “As soil alkalinity increases and the pH rises above 7.0, nutrients such as phosphorus, iron, boron, manganese, copper, zinc and potassium become chemically tied to the soil and less available for plant use.”

Lerner said that applying small amounts of wood ash to most soils will not adversely affect your garden crops, and the ash does help replenish some nutrients. “But because wood ash increases soil pH, adding large amounts can do more harm than good,” she said. “Keep in mind that wood ash that has been exposed to the weather, particularly rainfall, has lost a lot of its potency, including nutrients.”

“Specific recommendations for the use of wood ash in the garden are difficult to make because soil composition and reaction varies from garden to garden,” said Lerner. She explained that acidic soils will likely be improved by wood ash addition, slightly acidic soils should not be harmed by wood ash, but if your soil is neutral or alkaline, you should find another way to dispose of wood ash.

With that considered, what is a reasonable amount of wood ash to apply? Given that every garden is different, the first thing to consider is your soil’s pH. The more acidic, the more wood ash that can be applied. However, after researching this topic from several land-grant university sources, I found recommendations from 10-20 pounds of wood ash per 100 square feet to 5-25 pounds per 1000 square feet. Those differences do not inspire confidence in a blanket recommendation. Try what you believe is a reasonable amount based on your soil’s pH, and continue to monitor soil pH over time.

If you have dumped heavy amounts of wood ash on your garden annually without regard to soil pH, it’s likely your garden has become unproductive. Your best bet may be to move your garden location or construct raised beds filled with new topsoil.

Finally, Lerner cautioned against using wood ash on “acid-loving” plants. “Some plants, such as asparagus and juniper, are more tolerant of slightly alkaline conditions than 'acid-loving' plants, such as potatoes, rhododendrons and blueberries,” she said. “Wood ash should never be used on acid-loving plants.”

Alternative uses of wood ash would include spreading thinly over a lawn or using it as a “brown” in the compost pile. Use caution when topdressing a lawn because grass is also affected by soil pH.

Find Lerner’s original article on Purdue Consumer Horticulture's Yard and Garden News site.